Membrane fouling is one important problem in the membrane industry. It is characterized by a decrease in membrane permeation flux over time, which is generally induced by components in a feed solution passing through the membrane. It may be caused by molecule adsorption in the membrane pores, pore blocking, or cake formation on the membrane surface. A decrease in permeation flux increases operation energy use, and to overcome this, cleaning is required. However, this is only a temporary solution, and fouling typically decreases the life-span of the membrane.
As a method for reducing fouling of membranes for reverse osmotic pressure (RO), forward osmotic pressure (FO), ultrafiltration (UF), and microfiltration (MF), introduction of a hydrophilic surface to the membrane is a fundamental solution that is capable of providing fouling resistance while increasing the life-span of the membrane.
To increase fouling resistance of a membrane by graft polymerization of a hydrophilic group on the membrane surface, various hydrophilic monomers are grafted to various synthesis membranes to restrict fouling by microorganisms such as bacteria and the like and natural organic materials such as proteins and the like. An important drawback of the surface modification method is in initiation of graft polymerization using high energy gamma radiation or plasma. This approach may increase membrane manufacture cost, and it is not controlled well.
Another method for producing a fouling resistant surface is to manufacture a membrane including a microphase-separated polyacrylonitrile amphiphilic graft copolymer. Although it has drawbacks that a novel material should be developed through synthesis and a membrane manufacturing method should be established to manufacture a separation membrane, this method may minimize transformation of the membrane and impart long-term stability.
Another method for producing a membrane for fouling resistance is to introduce polymeric additives having hydrophilicity during manufacture of a membrane. Since this method does not require a processing step in the manufacture of a membrane, the cost is limited, and it is easily incorporated into the existing membrane casting process. To obtain uniform pore size, comparatively homogeneous polymers should be used, however, in this system, strong chemical bonding scarcely occurs, and thus the hydrophilic polymeric additives are discharged to deteriorate long-term stability.
The easiest method of producing a membrane for fouling resistance is to coat a hydrophilic material on the surface of a manufactured membrane. This approach is close to commercialization now, and has been actively researched.